1. Field of the Invention
This invention relates to an electronic transmission device and a signal transmission method using such electronic transmission device. More particularly, the present invention relates to an electronic transmission device that can avoid signal delay when connected in serial form to another electronic device even an authentication process is applied at each of the electronic transmission device.
2. Description of the Related Art
Conventionally in a computer apparatus or the like, a digital image signal is once converted into an analog image signal and then supplied to a display device by way of an analog transmission line. However, as a display device such as a LCD (Liquid Crystal Display) that is driven its display section by the digital signal is getting popular, a digital transmission method for driving the display device without converting the digital image signal into the analog image signal has become standard in order to avoid deterioration of image quality and reduction of cost.
FIG. 6 shows a construction chart of a digital connection, wherein two electronic devices such as a signal transmission device 100 and a signal reception device 130 are connected by way of a digital transmission line. Regarding the digital transmission line, the DVI (Digital Visual Interface) format is proposed by the DDWG (Digital Display Working Group), for example. In this DVI format, digitized three primary color image signals are serially transmitted color by color at base band transmission through the TMDS (Transition Minimized Differential Signals) high-speed transmission line developed by I2C Silicon Image Co. Further, a two-way transmission line such as the I2C (Inter Integrated Circuit) bus having a lower transmission speed than the TMDS high-speed transmission line (here-in after referred to as a high-speed serial bus) is provided, and this two-way transmission line (here-in after referred to as a two-way serial bus) is applied as a transmission line for the DDC (Display Data Channel) format in relating to the Plug & Play proposed by the VESA (Video Electronics Standards Association).
A transmission band for this high-speed serial bus of the DVI format can transmit image signal by using 3 channels (single-link) high-speed serial bus, as shown in FIG. 7, in a case when data amount of transmitting image signal is up to about 165 M-pixels/sec., namely an image display of a SXGA (Super Extended Graphics Array) (1280 pixels×1024 pixels) size and a vertical frequency of 85 Hz with a GTF (General Timing Formula) blanking is executed or an image display of an UXGA (ultra Extended Graphics Array) (1600 pixels×1200 pixels) size and a vertical frequency of 60 Hz with a GTF blanking is executed. However in a case when an image display of an UXGA (1600 pixels×1200 pixels) size and a vertical frequency of 75 Hz with a GTF blanking is executed, a 2-link high-speed serial bus is employed.
Contents data transmitted through such digital transmission line like the above-described can be easily and infinitely copied in a perfect form because of the digital signal transmission. This becomes a great merit for a user, but becomes serious problems for a provider who supplies copyrighted materials such as films with a legal protection. Because of this, it is proposed that in case of transmitting a digital signal of the contents data through the digital transmission line of the DVI format, an encryption for the copyright protection is executed and a decryption is made possible at a receiving side only if an authentication is properly conducted.
By the way, between two electronic device for legal protection, namely an authentication process is executed between a signal transmission device for outputting encrypted digital image signal of the contents and a signal reception device for receiving the signal from the signal transmission device as a pair, and if it is judged that an authorized device is connected, then key information for decrypting the encrypted digital image signal of the contents data is supplied. In this case, image display operations by the plural signal reception devices are executed in order to improve advertising effect, such plural signal reception devices are connected to multi-stage by a star connection as shown in FIG. 8. In this case, when the authentication is properly executed with the signal transmission device 100 and each of the signal reception devices 130, the key information is supplied to the signal reception device from the signal transmission device and it becomes possible to execute the image display by the signal reception device to which the key information is supplied.
If a relay function is added to the signal reception device, a plurality of signal reception devices 130 are connected to the signal transmission device 100 in a multistage daisy-chain connection as shown in FIG. 9.
FIG. 10 shows a conventional signal reception device to which a relay function is employed. A signal receiving section 132 of a signal reception device 130 is connected to a device of a signal transmitting side through a high-speed serial bus connected to a connector 131, and a signal transmitting/receiving section 133 is connected to a signal transmitting side through a two-way serial bus connected to the connector 131. Further a transmitting section 142 is also connected to the signal receiving side through the high-speed serial bus connected to the connector 141 and the signal transmitting/receiving section 143 is also connected to the signal receiving side through the two-way serial bus connected to the connector 141.
A decrypting section 151 is connected to the signal receiving section 132 and supplies an encrypted digital signal DGs transmitted from the signal transmitting side to a decrypting section 151. Further the signal transmitting/receiving sections 133, 143 are connected to an authentication process section 150 and executes an authentication process together with the signal transmitting side and the signal receiving side by way of the two-way serial bus.
In case of transmitting the encrypted digital image signal of the contents data for a legal protection, the authentication process is executed between the authentication process section 150 of the signal reception device and the signal transmitting side, and when key information Ka is supplied from the signal transmitting side to the authentication process section 150 and when authenticated as a connection of an authorized device, the authentication process section 150 supplies the key information Ka to the decrypting section 151.
The decrypting section 151 executes the decryption of the encrypted digital signal DGs supplied from the signal receiving section 132 by applying the key information Ka supplied from the authentication process section 150. The decryption is executed at the decrypting section 151, and thus obtained three primary color digital image signals DG are supplied to a display section 152 and image display is executed at the display section 152. Further the three primary color digital image signals DG obtained at the decrypting section 151 are also supplied to an encryption section 155.
When the relay for the digital image signal of the contents data is executed, key information is supplied from the authentication process section 150 to the encryption section 155 and an encryption of the three primary color digital image signals DG is executed by applying the key information supplied from the authentication process section 150 in the encryption section 155. A digital signal DGt generated at the encryption section 155 is supplied to the transmitting section 142, and is then supplied to the signal receiving side from the transmitting section 142 through the high-speed serial bus and the connector 141.
Further, in the authentication process section 150, the authentication process is executed with the signal receiving side connected to the connector 141, and if the authentication process indicates that the device is the authorized one, key information Kb for decrypting the encrypted digital signal DGt is also supplied to the signal receiving side.
Thus the image display is done by decrypting the received digital image signal DGs and further thus obtained digital signal is supplied to another signal receiving side by re-encrypting the decrypted signal DG. In this case, when it is judged by the authentication process with the signal receiving side that the authorized device is connected, then the key information Kb for decrypting the supplied digital image signal is transmitted to the signal receiving side. Accordingly, when a plurality of devices is connected in daisy-chain connection, it is possible to attain the legal protection properly.
In this case, when a plurality of signal reception devices is connected to a single signal transmission device as shown in FIG. 8, due to limitation of size and design of the signal transmission device, the number of connectors to be provided on the signal transmission device has a limitation, and there is a case where desired numbers of signal reception devices cannot be connected.
The encrypted signal is executed the image display after decryption and thus decrypted signal is supplied to another signal reception device after re-encrypting the decrypted signal as shown in FIG. 10, then the decryption and encryption process are to be done at every time when the signal is relayed. Accordingly, the digital image signal of the contents data is delayed at every time when relayed through one signal reception device by the decryption and encryption process, so that if relatively large number of signal reception devices are connected in the daisy-chain connection, the time delay is accumulated at the last connected signal reception device. In this case, the time difference of the displayed image between the display image by the signal reception device of the signal transmission device side and the display image by the signal reception device connected at the end of the daisy-chain connection becomes relatively large and thereby, the simultaneity of the image display cannot be maintained.